1. Field of the Invention
The present invention relates to endoscopic apparatuses and more particularly to piezoelectric-element drive mechanism control to drive the movable lens, arranged in an objective optical system, through expansion and contraction of a piezoelectric element for the purpose of auto-focusing and zooming functions.
2. Description of the Related Art
For example, the electronic endoscopic apparatus has an electronic endoscope (scope), a processor unit, a light source, a monitor and so on. In the tip of the electronic endoscope, there are arranged an objective optical system (lenses), a CCD (charge coupled device) serving as a solid-state imager, and so on. Depending upon the illumination of from the light source, the CCD takes an image of a subject-of-observation. The image signal is video-processed in the processor unit, to thereby display a video image of the subject-of-observation on the monitor. The objective optical system is built therein with a movable lens for the purpose of zooming and focusing functions. It is a recent practice to use a piezoelectric-element drive mechanism in order to drive such a movable lens.
In the piezoelectric-element drive mechanism, the drive shaft coupled to a piezoelectric element is frictionally engaged with a moving body so that the moving body can be moved slightly by delivering the expansion/contraction of the piezoelectric element to the moving body through the drive shaft (JP-B-4-52070 and Japanese Patent No. 3,635,525). By controlling the waveform, etc. of a drive voltage to be supplied to the piezoelectric element, the moving body can be moved in forward and backward directions. Such a piezoelectric-element drive mechanism is advantageously arranged in the tip of an endoscope having a reduced diameter because of its easiness to reduce the size.
However, in the movable lens movement control based on the conventional piezoelectric-element drive mechanism, when drive signals are provided in pulse count to drive the lens forward and backward equally in amount in the optical-axis direction, there encounters a difference of movement amount in between forward and backward movements thus resulting in a problem of the lowed accuracy of movement amount control. Namely, a difference occurs in moving amount in between the forward and backward due to the structure of the piezoelectric-element drive mechanism, such as a piezoelectric element as a drive source, a drive shaft coupled to the drive shaft, a frictional engagement state of between the drive shaft and the moving body. The difference depends also upon the characteristic unique to the piezoelectric-element drive mechanism, i.e. individual difference.